1. Field of the Invention
The present invention relates to turbine blade assemblies, and in particular, to assemblies reinforced with stays.
2. Description of Related Art
Independence from foreign energy sources is highly desirable. In particular, reliance on foreign crude oil as an energy source has tended to cause economic hardship and instability because the price of crude oil can vary dramatically based on a number of unpredictable geopolitical and economic factors.
In some regions wind power is a viable domestic energy source. Known wind-driven turbines employ rather long turbine blades that rotate an electrical generator. The entire wind turbine can be mounted on a high tower to provide clearance for large blades and to expose the blades to more dependable winds existing at a higher elevations.
In any event, the economic viability of the wind turbine depends greatly on the capital cost in comparison to the energy generated, which in turn depends greatly on the turbine's ability to capture the largest amount of energy.
For a given wind velocity, the power delivered by a turbine varies as the square of the diameter, i.e., doubling the diameter swept by the blades quadruples the energy captured. Consequently, designers have attempted to employ longer blades in order to increase the power output. For this reason, turbine designers have proposed blades that are a hundred feet in length.
The current designs of large horizontal wind turbines mostly utilize a three-blade configuration. To withstand strong gust of winds in extreme weather, the blades have to be made extremely strong. This directly increases the weight of the blades, resulting in heavier total head mass for the turbine systems, which translates into higher turbine system costs. Generally, as the diameters of the turbines increase, the blade weights increase exponentially. According to a paper by the National Renewable Energy Laboratory in 2001, the blade weight increase is proportional to the 2.4 power of the blade length with blades having a length of between 20 and 40 meters. Hence finding designs that effectively control the weight increase is critical for large wind turbines.
Blade tip speed is one of the major constraints that limit the size of the current three-blade wind turbine. At a typical fixed operating wind speed, if the diameter of a wind turbine increases, the tip speed ratio of the turbine blade will increase accordingly (Tip speed ratio=Tip speed/Working wind speed). The increase of tip speed ratio will result in the decease of turbine efficiency. To reduce the tip speed ratio, then it is necessary to increase the number of blades of the turbine in order to lower the operating rpm of the turbine. However, increasing the number of blades will increase the massiveness of the turbine and the turbine weight, so that attempts to increase the number of blades is difficult with today's design. The solution to this hurdle is again a new design that can slim down the size of the blades and reduce the blade weight.
Current designs of large wind turbine blades emphasize the use of composite materials in order to meet the requirement of blade strength. Composite materials are relatively new and are not that well understood compared with the traditional material like metals. Also, the composite materials usually require special manufacturing processes that are more costly.
In the past, attempts had been made to utilize cables to reinforce the turbine blades. But these known designs attach the cables to the blades in a rudimentary way (e.g., U.S. Pat. No. 4,403,916), resulting in blades which cannot be rotated around their axes to adjust pitch. So pitch controllers cannot be used in such designs. Such pitch control can be helpful in today's large modern wind turbines for maintaining a fixed rotating speed in the variable operating wind speed environment.
See also U.S. Pat. Nos. 440,266; 766,219; 1,790,175; 2,103,910; 2,516,576; 4,297,076; 4,403,916; 4,729,716; 6,155,785; and 6,320,273.